1. Field of the Invention
This invention relates to a surface acoustic wave filter comprising at least one comb shaped interdigital electrode mounted on a piezoelectric substrate, particularly a surface acoustic wave device which can be made smaller along the direction of propagation of surface acoustic waves than that of the prior art.
This application is based on Japanese Patent Application No. Hei 11-318105, the contents of which are incorporated herein by reference.
2. Background Art
Conventionally, in a surface acoustic wave (referred to as xe2x80x9cSAWxe2x80x9d hereinafter) filter, because the input and output comb shaped interdigital electrodes (referred to as xe2x80x9cIDTxe2x80x9d hereinafter) face each other, the dimension along the direction of propagation of surface acoustic waves must be larger than the sum of the lengths of the input and output IDTs, and miniaturization of the SAW filter is restricted. FIG. 4 shows an example of the configuration of a transversal (or transmission) SAW filter of the prior art. In this configuration, the size along the left and right direction of a chip 21, that is, the direction of propagation of surface acoustic waves, is inevitably larger than the sum of the lengths of the input IDT 22 and the output IDT 23, and therefore the miniaturization of the chip is restricted.
In another prior art there is a design for miniaturizing a chip by reflecting surface acoustic waves obliquely by means of an oblique metal grating reflector. FIG. 5 shows a prior art Published Japanese translation No. Hei 5-501486 of PCT International Publication in which oblique metal grating reflectors 34 and 35 are mounted on a transversal SAW filter to reflect a SAW. In this configuration, the dimension along the left and right direction of the chip (direction of propagation of the SAW) can be reduced by the way in which the SAW is reflected. However, because the absolute value of the reflection coefficient of a metal reflector is generally very small, many metal reflectors are usually necessary to achieve a sufficient amount of reflection, and as a result the miniaturization of the chip is restricted. In this configuration, when the number of oblique metal grating reflectors is reduced in order to miniaturize the chip, a problem results in which there is a large loss in reflection. On the other hand, there is also a problem in that when there is a large number of oblique metal grating reflectors, the chip""s dimensions must be large.
In another prior art, a cut surface of a chip is used as a reflection edge surface in a resonator type SAW filter. In FIG. 6, the reflection of a SAW from reflection surfaces 43 and 44, which are cut surfaces of the left and right edges of a chip, is utilized and standing waves are formed along the direction of the arrows in the figure, such that the SAW filter functions as one port resonator. In this configuration, a required condition to form a resonator is that the SAW incident on the reflection surface 43 and the SAW required must be in phase. However, in order to put them in phase, the distance between the reflection surface (cut surface) and an electrode 42 must be formed with very high precision (at the micrometer level), resulting in the problem that the chip is difficult to produce.
The purpose of the present invention is to provide a SAW filter in which a transversal type IDT, which does not resonate in response to the response the generation and reception of a SAW, is used, and in which it is not necessary that the distance between a reflection edge surface and an electrode be formed with very high precision; therefore the formation of a reflection surface is simple, unused regions on a chip can be reduced in comparison with the prior art, and a SAW filter, provided with a small chip which is a constituent element of a SAW device, can be manufactured
A transversal type surface acoustic wave filter as claimed in claim1 comprises: an input interdigital transducer (IDT), which is comprised of a pair of comb electrodes whose respective electrode fingers are interdigitally formed; substrate boundary surfaces which formed at an angle to a wave front of a propagating surface acoustic wave generated in said input IDT; an output IDT, which is comprised of a pair of comb electrodes whose respective electrode fingers are interdigitally formed; wherein said input and output IDTs and said substrate boundary surfaces are all formed on a piezoelectric substrate; and wherein said input and output IDTs are arranged in parallel and in close proximity to each other along the longitudinal direction of the electrode fingers of said IDTs, and are arranged such that the input and output sides of each IDT are oriented in a direction perpendicular to the longitudinal direction of said electrode fingers.
A transversal type surface acoustic wave filter as claimed in claim 2 is characterized in that said substrate boundary surfaces are step shaped, and the height of the step d of said step shaped substrate boundary surfaces is xcex less than d less than 5xcex, where xcex is a wavelength of the SAW.
A transversal type surface acoustic wave filter as claimed in claim 3 is characterized in that said substrate boundary surfaces are formed at an angle of 45 degrees to the wave front of said surface acoustic wave, and said surface acoustic wave only has displacement components parallel to said substrate.
A transversal type surface acoustic wave filter as claimed in claim 4 is characterized in that said surface acoustic wave, which only has displacement components parallel to said substrate, is a BGS wave.
A transversal type surface acoustic wave filter as claimed in claim 5 is characterized in that said surface acoustic wave, which only has displacement components parallel to said substrate, is a LOVE wave.
A transversal type surface acoustic wave filter as claimed in claim 6 is characterized in that said surface acoustic wave, which only has displacement components parallel to said substrate, is an SH wave.
A transversal type surface acoustic wave filter as claimed in claim 7 is characterized in that said surface acoustic wave, which has only displacement components parallel to said substrate, is an SSBW.